But at what point does the present moment become the past, and a new moment becomes the present?
This was supposed to be a rhetorical question. I’m not sure anyone really knows the answer. If you think in terms of electronics and computer technology, a moment of time can be almost infinitesimally small. On the other hand, if you think in terms of geological or astronomical time, what seems like eons to us is insignificant. So it all boils down to perception and relativity.
Time is pervasive. All existence, every sensory perception, thought, and action takes place within the matrix of time and space. Time is part of the fabric of reality, or of maya, depending on what you believe. Our experience of gross reality takes place in the present time, the eternal now. Our more subtle nature, the mind, is not limited to the present. But we do not experience past and future in the same way that we experience reality. We know our own past as memory. We can also know the past as knowledge that we learn from sources outside ourselves. All learned knowledge is knowledge of the past. Likewise our mind cannot experience the future, it can only imagine what the future will be. A point of time in the future can take any form we desire until that point in time becomes the present. The present determines what that point in time and space will remain, until it is forgotten. Ultimately, experience is ephemeral.
To be here now means simply to restrict our mind from memories and imaginations, and focus it on the present. It is only in the present that we can see or hear or feel or think or know anything, or experience the presence of another.